DESCRIPTION OF THE PRIOR ART
The desire to produce potable or otherwise usable water from sea water and brackish water has existed for many years. Considerable technology now exists with the capability to purify sea water and brackish water, both of which may be referred to herein as "saline water" or as "salt water." One known technique of desalination is the reverse osmosis process which relies on the use of high pressure to force relatively salt-free water through membranes thereby separating relatively salt-free water from the initial saline water. Another known process is the evaporation process which requires heat to evaporate the water and thereby separate the water in the form of condensate from the saline water. In addition, brine concentrators such as Resources Conservation Co. brine concentrators (R.C.C. brine concentrators) are used to separate water and salt.
Each of these techniques relies on energy to provide the means for separating the water from the salt. For example, with some reverse osmosis membranes it is possible to obtain from sea water a reasonable yield of product water containing less than 500 ppm of dissolved salts. The salts contained in the saline water are substantially rejected by the membrane and are concentrated in a reject brine. The reverse osmosis process necessarily requires energy. The amount of energy required and the yield of product water produced from saline water are directly related to the salt concentration of the saline water being processed and, within limits, to the temperature of the saline water. In general, reverse osmosis units operate less efficiently and require more energy per unit of water product as the salt concentration of the saline water feed increases. In fact, at very high salt concentrations reverse osmosis should not be employed. However, energy costs are a major factor in determining whether a desalting facility is or is not economic.
On the other hand, the energy required to produce water from sea water by multi-stage flash distillation, which inherently produces water of about 10 ppm of dissolved salts, is much greater than that required to produce the same amount of water by reverse osmosis. However, multi-stage flash distillation can process saline water of higher salt content than can be effectively processed by reverse osmosis. Within its zone of capability multi-flash distillation requires approximately the same amount of energy per unit of distillate water product whether the salt content of the feed water is low or high. Thus as the salt content of the feed water decreases, more product water of 500 ppm salt content can be obtained by blending more and more saline feed water with a given quantity of distillate.
Efforts have been made to combine desalination systems and rely on combinations of desalination processes to make an efficient and economically acceptable system. One such system is shown in U.S. Pat. No. 3,632,505 (Nelson, Jan. 4, 1972). The system disclosed therein relies basically on arranging reverse osmosis and flash evaporation units essentially in parallel. High-pressure steam turbines provide the pressure for the reverse osmosis system and the high-pressure turbine discharge is used to provide the heat for the evaporation units.
Attempts have also been made to recover by-products from the desalination of brackish water. As set forth in the Fourth International Symposium on Salt -- Northern Ohio Geological Society a process is outlined for the partial deionization of brine to produce by-products such as sodium carbonate and calcium chloride and waste such as calcium sulfate, calcium carbonate and magnesium oxide.